... except Audacity.h; and in no others.
Do so even if Experimental.h gets multiply included, as in both the .h and
.cpp files.
This makes it easier to do a text scan to be sure there are no unintended quiet
changes of meaning because of omission of Experimental.h when the flag is
an enabled one.
Also move inclusions of Experimental.h earlier.
Also don't require Experimental.h to be preceded by Audacity.h to define
EXPERIMENTAL_MIDI_OUT correctly.
... it's either the source of the connection that is being destroyed, or other
object (such as an ancestor window) transitively owning it and so causing it to
be destroyed too;
or, the sink is being destroyed, and that sink is a wxEvtHandler (which is
always so for Disconnect, though not for Unbind in case Bind was passed a
member function of a non-wxEvtHandler).
wxWidgets takes care of erasing the connection in such cases.
This removes most calls to Disconnect and Unbind. Many destructors shrank to
nothing.
Notably, in case of popup menu handling, the call to Disconnect is not removable
because the object being destroyed is neither the source nor the sink.
... Should have no effect on generated code, except perhaps some slight faster
virtual function calls. Mostly useful as documentation of design intent.
Tried to mark every one of our classes that inherits from another, or is a
base for others, or has abstract virtual functions, and a few others besides.
